1. Field of the Invention
The present invention relates to a supercharged internal combustion engine, and more particularly to a supercharged internal combustion engine wherein an air-fuel ratio on the lean side with respect to the theoretical (stoichiometric) air-fuel ratio is obtained when the engine is in a supercharging state.
2. Description of the Related Art
In a lean burn internal combustion engine using gasoline as the fuel for example, as disclosed in Japanese Unexamined Patent Publication No. 58-59327, a lean air-fuel ratio value with respect to the theoretical air-fuel ratio is obtained. To obtain a stabilized combustion of the lean air-fuel ratio, a stratification of the combustible mixture is created in a combustion chamber so that a rich portion is formed adjacent to the electrodes of a spark plug, which rich air-fuel mixture portion is easily ignited, and a flame thus generated is extended to cause a burning of a lean portion located around the rich portion. In such a known type lean burn system, a lean air-fuel mixture value of the air-fuel mixture of between 18 to 22 is obtained when the engine is under a low load. When the engine load is increased to a medium load zone, a combustible mixture of a theoretical air-fuel ratio or a lean air-fuel ratio near to the theoretical air-fuel ratio, of between 15 to 17, is formed to obtain a desired engine performance. When the engine load is in a high load zone, such a theoretical air-fuel ratio or lean air-fuel ratio near to the theoretical air-fuel ratio of between 15 to 17 allows an easy increase of the exhaust temperature, and thus a catalytic converter is apt to be excessively heated. To prevent an excessive increased of the exhaust temperature, the air-fuel ratio is controlled to a rich air-fuel ratio of, for example, between 10 to 11. See Japanese Examined Patent Publication No. 62-54977.
In the prior art stratified lean combustion engine, a rich air-fuel ratio is obtained at a engine high load operation, to prevent an excessive heating of the catalytic converter, but this rich air-fuel mixture lowers the fuel consumption efficiency.
The excessive temperature of the catalytic converter is prevented if an air-fuel ratio of, for example, 20, which is outside the range wherein the excessive exhaust gas temperature occurs, is used, but such an extremely lean air-fuel ratio, although not providing an excessive increase of the temperature, easily causes the combustion of the combustible mixture to become unstable, which lowers the engine output performance.